Mentor: Annette Bollmann, Ph.D.

Nitrification, a critical process in the nitrogen cycle, involves the microbial conversion of ammonia to nitrate, primarily carried out by ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), and complete ammonia oxidizers (comammox). Unlike other ammonia oxidizers, comammox organisms can fully oxidize ammonium to nitrate via nitrite. While AOA and comammox are typically found in oligotrophic environments with low ammonium availability, their interactions with heterotrophic bacteria remain poorly understood. Two enrichment cultures—BO1 and BO4—were established from the freshwater Lake Burr Oak and contained the ammonia oxidizers Nitrosoarchaeum sp. BO1 (AOA) and Nitrospira sp. BO4 (comammox). The competition for ammonium between these ammonia oxidizers was investigated under ammonium-limiting conditions and with initial ammonium concentrations of 50 and 500 µM. Under all tested conditions, the comammox strain Nitrospira sp. BO4 outcompeted the AOA (Ghimire-Kafle et al., 2024). The heterotrophic communities associated with the ammonia oxidizers were determined via 16S rRNA amplicon sequencing to analyze the shifts in those communities associated with the competition between the AOA and comammox cultures. In all competition experiments, comammox won the competition between the two ammonia oxidizers. The same picture was detected in the heterotrophic communities. The heterotrophic bacteria dominant in the comammox enrichment culture became dominant in the mixed cultures together with the comammox strain. In the chemostat experiments, the heterotrophic community shifted towards the comammox-dominated community even if before introducing comammox, AOA and their heterotrophic community were dominant. This indicates that the dominant ammonia oxidizers shaped the associated heterotrophic community.

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